In this case regeneration and reproduction coincide; there is no regeneration except the origin of a new individual from a reproductive cell. [Illustration: FIG. 35 _B_ (repeated). _Hydra viridis_, the Green Freshwater Polyp. Section through the body-wall, somewhere in the direction of _ov_ in Fig. 35 _A_. _Eiz_, the ovum lying in the ectoderm (_ect_), and including zoochlorellæ (_schl_) which have immigrated from the endoderm (_ent_) through the supporting lamella (_st_). After Hamann.] Let us now ascend to the lowest of the Metazoa, for instance, the freshwater polyp, Hydra (Fig. 35 _A_), and we find a high degree of regenerative capacity in the restricted sense, for, in addition to the power of producing germ-cells, that is, cells which, when two combine in amphimixis, give rise again to a new animal, almost any part of the polyp can regrow a whole animal. Not only has Hydra been cut in from two to twenty different pieces, but it has even been chopped up into innumerable fragments, and yet each of these, under favourable circumstances, was able to grow again into a complete animal. Nevertheless, we are not justified in concluding that every cell possesses the power of reproducing the whole. If, with the help of a bristle, we turn one of these polyps outside in like the finger of a glove, and then prevent it turning right again by sticking the bristle transversely through it, it does not live, but soon dies, obviously because the cells of the two layers of the body, ectoderm and endoderm, cannot mutually replace each other, and cannot mutually produce each other. The inner layer, now turned outwards, cannot resist the influence of the water, and the outer layer, now turned inwards, cannot effect digestion; in short, one cannot be transformed into the other, and we must therefore conclude that both are specialized, that they no longer contain the complete germ-plasm, but only the specific determinants of ectoderm and endoderm respectively. The animal's high regenerative capacity must therefore depend on the fact that certain cells of the ectoderm are equipped with the complete determinant-complex of the ectoderm, in the form of an inactive accessory idioplasm, which is excited to regenerative activity by the stimulus of wounding, and that, in the same way, the cells of the endoderm are equipped with the whole determinant-complex of the endoderm. It need not be decided whether all or only many of the cells, perhaps the younger ones, are thus adapted for regeneration; in any case a great many of them must be distributed throughout the whole body, with perhaps the exception of the tentacles, which are by themselves unable to reproduce the whole animal. When the animal is mutilated, the cells of both layers, equipped with their respective determinant-aggregates, co-operate in reproducing the whole from a part.